Hopefully we will see it in the future once more details are added to the game.
Pravend was a capital of the Empire at some point, perhaps when they conquered the land around it, but then they sort of abandoned that part of the continent as they moved deeper, it was basically ruins when the Vlandians settled there, otherwise it would be owned by Derhert and they would have made it their capital.
It is quite hard to realize which is the true cultural capital of the Empire, or at least the bigger city.
-We know Lycarion was the seat of Arenicos during his rule, but we dont know whether he already owned it beforehand, or if he ruled Onira and was given Lycarion due to it being the capital of the Empie. I guess the low prosperity at the start of the game is due to the riot after Arenico's death.
-My second option was one of Lucons' settlements, since in theory one of those was settled the Senate, but neither of those have 2 villages attached to it, their prosperity at the beggining of the game aint great, and neither have Tier 3 walls.
- The more prosperous town at the whole Empire is Ortysia, but it doesnt have T3 walls.
-Sanopea has T3 walls and high prosperity, plus it is owned by clan Neretzes, so it was probably Emperor Neretzes capital. (It is weird that their formed figurehead was killed just at the rebellion, to then be succeded by someone that supported Lucon...)
-Zeonica is probably one of my best bets, it is at the coast, it has 3 towns attached, it is well defended by both sides, T3 walls and pretty high prosperity.